In order to prevent rear wheel lifting caused by sudden brake operation with respect to a front wheel in a two-wheeled motor vehicle, there is known, for example, the technology described in patent document 1 below.
The technology in patent document 1 utilizes the fact that, when a wheel angular acceleration α is 0, brake force expressed as a predetermined function of a wheel cylinder pressure (caliper pressure) and road surface frictional force expressed as a product of a road surface μ and a vertical load W match to estimate and calculate vertical loads WFR and WRR acting on the front and rear wheels on the basis of a front wheel cylinder pressure and a rear wheel cylinder pressure that have been detected. In this estimation calculation, the technology utilizes the fact that the sum of the vertical loads WFR and WRR is constant to eliminate the road surface μ that is an unknown. Additionally, when it has been judged that, during brake operation of the front wheel, the vertical load WRR on the rear wheel that has been estimated has fallen below the matching value, the technology reduces the front wheel cylinder pressure (caliper pressure) to thereby appropriately alleviate the drop in the vertical load WRR on the rear wheel.
However, in the above-described technology, under a condition where the assumption that the wheel angular acceleration α is 0 is not established, such as when the wheel cylinder pressures abruptly change or when the wheels actually skid, there is the potential for the estimated value of the vertical load WRR on the rear wheel to become inaccurate; thus, under such a condition, there is the fear that prevention of rear wheel lifting will no longer be certain.    Patent Document 1: JP-A-5-77700